1. Field of the Invention
The present invention relates to a method for preparing sulfide phosphors and, in particular, to a method for preparing sulfide phosphors of high luminance by industrial mass-production by efficiently controlling the firing atmosphere in preparing the sulfide phosphors.
2. Discussion of Background
Phosphors composed of a host material of a sulfide of a formula (Zn.sub.1-x Cd.sub.x)S where 0.ltoreq..times..ltoreq.1, such as zinc sulfide (ZnS) or zinc cadmium sulfide ((Zn,Cd)S), as activated by an activator of silver (Ag), copper (Cu) or gold (Au) are practically used in color picture tubes or fluorescent display tubes and have an extremely long history. Phosphors composed of a host material of an alkaline earth metal sulfide, such as calcium sulfide (CaS) or strontium sulfide (SrS), as activated by an activator of a rare earth element, such as cerium (Ce), europium (Eu), samarium (Sm), terbium (Tb), dysprosium (Dy) or thulium (Tm) are also known as regenerative phosphors from the past and, additionally, they are being investigated for use in electroluminescent (EL) devices in these days. Since the sulfide phosphors ("sulfide phosphors" as referred to herein indicate any and every phosphor having a sulfide as a host material thereof, including the above-mentioned phosphors) have a sulfide as its host material, firing thereof is to be effected in a sulfurizing atmosphere or in a neutral atmosphere for the purpose of preventing them from being oxidized. In general, such firing is effected in a sulfurizing atmosphere. In the past, the raw material for the phosphor was fired in a hydrogen sulfide stream. However, hydrogen sulfide is toxic and the handling thereof is difficult. Therefore, a different method has been employed where the raw material for a phosphor is blended with carbon and sulfur, or carbon and sulfur are directly brought into contact with the raw material mixture for a phosphor and the resulting raw material mixture is placed in a covered heat-resistant container and heated therein whereby the mixture is fired in a sulfurizing atmosphere containing carbon sulfide to be formed by the thermal reaction between the carbon and sulfur in the said heat-resistant container (U.S. Pat. Nos. 3,595,804 and 2,623,858). Apart from this, another method of using an organic substance such as saccharides which contain a large amount of carbon atoms as the constituting component and which may easily be converted into carbon oxide gases by firing has been proposed but has not yet been put to industrial or practical use.
Where sulfide phosphors are prepared in a sulfurizing atmosphere, if the sulfurizing atmosphere is lost even temporarily in the firing step, the phosphor would be oxidized. As a result, a sulfide phosphor of high luminance could not be obtained by the process. Therefore, it is especially essential to continuously maintain the sulfurizing atmosphere around the raw material mixture being fired during the period of from the middle stage of firing to the latter stage thereof while the raw material mixture for the phosphor is allowed to stand at a determined temperature for a determined period of time so as to promote the interaction of the components of the raw material mixture to finally produce the intended phosphor.
However, in accordance with the known method where the raw material mixture for phosphor is fired in the presence of carbon and sulfur to give the necessary sulfurizing atmosphere and a sulfide phosphor of high luminance is prepared from the mixture, it was found that a large amount of the sulfur evaporates and almost all the incorporated sulfur disappears in the initial firing stage since the boiling point of sulfur is far lower than the firing temperature of the sulfide phosphor and, as a result, the sulfur necessary for maintaining the sulfurizing atmosphere is lacking and a sufficient sulfurizing atmosphere could not be maintained during the period of from the middle stage of firing to the latter stage thereof while the intended phosphor is being produced. Specifically, it was found that, in the known method of preparing a sulfide phosphor where the raw material mixture for the phosphor is placed in the firing container along with carbon and sulfur and is fired therein, there is a noticeable time lag between the stage where the sulfurizing atmosphere is most required for the production of the phosphor and the stage where the incorporated carbon and sulfur are reacted to give a sufficient sulfurizing gas atmosphere. Accordingly, it is necessary that a large amount of carbon and a large amount of sulfur are to be placed in the firing container along with the raw material mixture for phosphor for the purpose of maintaining the sufficient sulfurizing atmosphere during the step of producing the phosphor from the raw material mixture thereof. In such case, however, the proportion of the raw material mixture for phosphor in the firing container would be relatively smaller than the enlarged amounts of the carbon and sulfur therein and, as a result, there is a problem of lowering the producibility of the phosphor product. Accordingly, such problem is desired to be overcome.